Alloyed steel powder with improved degree of sintering for metal injection molding and sintered body

ABSTRACT

An alloyed steel powder for metal injection molding that eliminates the problems of decreased product strength and difficulty of temperature control which exist in conventional alloys for sintering and that improves productivity of the sintering furnace is provided, together with a sintered body thereof. This alloyed steel powder for metal injection molding consists, as mass percentages, of 0.1 to 1.8% C, 0.3 to 1.2% Si, 0.1 to 0.5% Mn, 11 to 18% Cr, 2 to 5% Nb and the remainder Fe and unavoidable impurities, and which may further comprise 5.0% or less of at least one of Mo, V and W, or a sintered body (wherein C is 0.1 to 1.7%) of these powders. The alloyed steel powder for metal injection molding of the present invention results in a sintered body with a constant sintered density over a 50° C. range of sintering temperatures, thereby facilitating sintering temperature control and improving productivity.

TECHNICAL FIELD

The present invention relates to an alloyed steel powder for metalinjection molding (MIM) which is effective in making complex-shapedparts of very hard, highly corrosion resistant martensite stainlesssteel or tools of alloyed steel with a good dimensional precision, andrelates to a sintered body.

BACKGROUND ART

SKD11, SUS420, SUS440C and the like have conventionally been used asmetal injection molding powders for obtaining very hard, highlycorrosion-resistant sintered bodies. These steels, in which the hardnessis obtained mainly by the use of Cr carbide, exhibit an austenite phasein the sintering temperature range, and have a poor degree of sinteringbecause the speed of elemental movement (diffusion) which promotessintering is slower than in a ferrite phase. On the other hand, if thetemperature is raised to the temperature at which a liquid phase appearsin order to promote sintering, a large amount of liquid phase arises atonce, carbides are formed as networks at the grain boundaries, andeither the strength of the product is seriously diminished or it isdeformed to the point that the shape of the product cannot bemaintained. To avoid these, it is necessary to proceed with thesintering temperature controlled within an extremely narrow temperaturerange of ±5° C. or in other words about 10° C. Because of this, it hasbeen necessary to limit the usable region of the sintering furnace,sacrificing productivity.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to eliminate the aforementioneddiminishment of product strength and difficulty of controlling sinteringtemperature which are problems of the aforementioned conventionalsintering alloys, and to provide an alloyed steel powder for metalinjection molding and a sintered body which contribute to enhancing theproduct's characteristics and improving the productivity of thesintering furnace.

In order to solve the aforementioned problems, the present invention hasthe following constitution.

(1) An alloyed steel powder for metal injection molding with an improveddegree of sintering, consisting, as mass percentages, of 0.1 to 1.8% C,0.3 to 1.2% Si, 0.1 to 0.5% Mn, 11.0 to 18.0% Cr, 2.0 to 5.0% Nb, and aremainder of Fe and unavoidable impurities.

(2) An alloyed steel powder for metal injection molding with an improveddegree of sintering, consisting, as mass percentages, of 0.1 to 1.8% C,0.3 to 1.2% Si, 0.1 to 0.5% Mn, 11.0 to 18.0% Cr, 5.0% or less of atleast one of Mo, V and W, 2.0 to 5.0% Nb, and a remainder of Fe andunavoidable impurities.

(3) An alloyed steel powder for metal injection molding with an improveddegree of sintering according to (2) above, wherein the amount of the atleast one of Mo, V and W is 0.3 to 1.6%.

(4) An alloyed steel sintered body for metal injection molding with animproved degree of sintering, consisting, as mass percentages, of 0.1 to1.7% C, 0.3 to 1.2% Si, 0.1 to 0.5% Mn, 11.0 to 18.0% Cr, 2.0 to 5.0%Nb, and a remainder of Fe and unavoidable impurities.

(5) An alloyed steel sintered body for metal injection molding with animproved degree of sintering, consisting, as mass percentages, of 0.1 to1.7% C, 0.3 to 1.2% Si, 0.1 to 0.5% Mn, 11.0 to 18.0% Cr, 5.0% or lessof at least one of Mo, V and W, 2.0 to 5.0% Nb, and a remainder of Feand unavoidable impurities.

(6) An alloyed steel sintered body for metal injection molding with animproved degree of sintering according to (5) above, wherein the atleast one of Mo, V and W is 0.3 to 1.6%.

The focus of the present invention is on producing a Nb carbide with alow diffusion by adding Nb to a steel alloyed primarily with Cr carbide.Because this Nb carbide has a low diffusion speed, it is unlikely tobulk by diffusion during sintering of the metal injection moldedproduct, and the Cr carbide is also precipitated around the core of thisNb carbide.

Using the pinning effect of this Nb carbide, it is possible to preventbulking and network formation of the carbide more effectively than whenonly Cr carbide is present.

In the constitution of the present invention, C forms carbides andcontributes hardness, resulting in a martensite structure. 0.1 to 1.8%is desirable as the amount range of C in the powder. The sinteringtemperature and sintered density vary according to the amount of C.Consequently, graphite is added appropriately during the molding of thepowder to adjust the amount of C in the sintered product to 0.1 to 1.7%.A sintered body with a high sintered density can be manufactured underan easy temperature control. The lower limit of 0.1% in both the powderand sintered body was set because that is the minimum amount necessaryto produce the aforementioned Nb carbide and because it is the minimumamount at which the C would dissolve in the matrix to form martensite.The upper limits of 1.8% in the powder and 1.7% in the sintered bodywere set considering the amount of C that is lost from the powder duringsintering because at this level C contributes to hardness by forming aCr carbide in the sintered body, but in an amount above 1.7% hardness isnot further improved and, to the contrary, toughness (transverse rupturestrength) is diminished.

Si improves deoxidation and hot water flow. If the amount is less than0.3%, the oxygen amount rises and hot water flow is adversely affected,while if it is more than 1.2%, hardenability is adversely affected.

If Mn is less than 0.1%, hot water flow is adversely affected, while ifit is over 0.5%, it combines with oxygen, increasing the amount ofoxygen in the powder. Consequently, it is set in the range of 0.1 to0.5%.

Cr improves hardenability and increases hardness by producing carbides.It also dissolves in the matrix including the carbides, thereby, itimproves corrosion resistance. A range of 11.0 to 18.0% is desirable.

Mo, V and W produce carbides, and together with Nb have a pinning effecton the Cr carbides, therefore, they enhance the strength and hardness ofthe sintered body. If these are more than 5.0%, toughness will bediminished so 5.0% or less is desirable, and a range of 0.3 to 1.6% ismore desirable from the standpoint of hardenability andcost-effectiveness. A noticeable improvement in hardness is difficult toachieve below 0.3%, while more than 1.6% is not cost-effective.

Nb controls diffusion of Cr carbides and improves hardenability by meansof the pinning effect of low-diffusion Nb carbides. By adding 2.0 to5.0% Nb, it is possible to improve the productivity of the sinteringfurnace because the sintering temperature needs only to be controlledwithin ±25° C. rather than within ±5° C., as is required conventionally.This effect isn't sufficient if the amount of Nb is less than 2.0%,while if it exceeds 5.0%, the amount of oxygen increases and themoldability is adversely affected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a pattern of sintering performed in an example of thepresent invention.

FIG. 2 is a graph of the sintering characteristics of SKD11.

FIG. 3 is a graph of the sintering characteristics of SUS420.

FIG. 4 is a graph of the sintering characteristics of SUS440C.

FIG. 5 is a graph of the sintering characteristics of ComparativeExample 1.

FIG. 6 is a graph of the sintering characteristics of Example 1 of thepresent invention.

FIG. 7 is a graph of the sintering characteristics of Example 2 of thepresent invention.

FIG. 8 is a graph of the sintering characteristics of Example 3 of thepresent invention.

FIG. 9 is a graph of the sintering characteristics of Example 4 of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The samples shown in Table 1 below were prepared and their sinteringcharacteristics tested.

TABLE 1 Steel Composition (%) Dm T/D type C Si Mn Cr Mo V W Nb O Fe (μm)(g/cm³) SKD11 1.66 0.34 0.44 11.80 1.02 — — — 3300 Remainder 11.90 4.04SUS420 0.27 0.85 0.33 13.09 0.59 — — — 3200 Remainder 10.01 4.30 SUS440C0.96 0.91 0.18 17.12 0.05 0.07 — — 2700 Remainder 9.72 4.21 Comp. 0.600.73 0.47 12.53 1.49 — — 0.34 3900 Remainder 10.22 4.27 Example 1Example 1 1.03 0.92 0.22 17.01 — — — 3.01 4100 Remainder 9.92 4.17Example 2 0.66 0.88 0.44 12.18 1.42 — — 3.22 4200 Remainder 10.98 4.18Example 3 0.96 0.87 0.21 17.12 0.41 0.17 0.08 2.99 3400 Remainder 9.864.08 Example 4 0.56 0.93 0.31 12.34 0.50 — — 2.81 2500 Remainder 9.924.17 Comp. 0.65 0.89 0.45 12.15 1.46 — — 7.33 13500  Remainder 10.344.20 Example 2

The C amount of each sample was adjusted. Graphite powder was added withthe aim of achieving C amounts after sintering of 1.30%, 1.50% and 1.70%for SKD11, 0.30%, 0.50%, 0.70% and 0.90% for SUS420, 1.30% for Example1, 0.75%, 0.80%, 1.00% and 1.20% for SUS440C, 0.50%, 0.70% and 0.90% forComparative Example 1 and Example 2, 1.30% for Example 3 and 0.90% forExample 4. A sintering test was not performed in the case of ComparativeExample 2 because the amount of oxygen was too great at the powderstage.

The sintering test was performed as follows.

A suitable amount of graphite powder was added to each of the metalpowders shown in Table 1, based on the target amount of C aftersintering, 5.0 wt % of stearic acid (outer number) was added to thepowder, and uniform kneading was performed with heating at 80° C.

The kneaded products were cooled to room temperature, and the solidifiedpellets were pulverized. The pulverized pellets were press molded at amolding pressure of 0.6 Ton/cm² (Ø11.3×10t, N=3).

Sintering was performed according to the pattern shown in FIG. 1. InFIG. 1, the sintering temperatures were the appropriate temperaturesshown in Tables 2 through 5, such as 1370° C., 1390° C. and 1410° C.

Tables 2 through 5 show the sintered density of each sample at differentsintering temperatures and for different target amounts of carbon aftersintering. The amounts of C, 0 and N in the sintered products of eachsample are shown at the bottom of Tables 2 through 5, along withsintered hardness (Hv) in the case of Tables 4 and 5. The sinteringcharacteristics shown in Tables 2 through 5 are also shown in graph formin FIGS. 2 through 9.

The structures were observed and the hardness of the sintered bodies wasmeasured to determine the respective appropriate control ranges ofsintering temperature. The appropriate control range of sinteringtemperature was the sintering temperature range within which thesintered density remained nearly constant within a range of ±0.1 g/cm³as the sintering temperature rose on the sintering temperature-sintereddensity graph.

TABLE 2 SKD11 SUS420 Target C amount (%) Target C amount (%) aftersintering after sintering Steel type 1.30 1.50 1.70 Steel type 0.30 0.500.70 0.90 Molded product 4.91 4.90 4.88 Molded product 4.85 4.81 4.784.76 Density density Sintering 1220 — — 6.84 Sintering 1250 — — 6.757.07 Tempera- 1230 — 6.71 7.25 Tempera- 1270 — — 6.82 7.47 ture (° C.)1240 6.81 7.20 7.61 ture (° C.) 1290 — — 7.06 7.78 1250 7.21 7.58 7.691310 6.82 — 7.38 7.91 1260 7.68 7.70 7.69 1330 6.84 6.98 7.79 — 12707.71 7.69 — 1350 6.86 7.27 7.85 — — — — — 1370 6.92 7.70 — — — — — —1390 7.41 7.69 — — — — — — 1410 7.70 — — — C (%) 1.28 1.47 1.66 C (%)0.33 0.57 0.79 0.99 O (ppm) 11 10 11 O (ppm) 17 40 27 41 N (ppm) 7 8 9 N(ppm) 3 4 1 3

TABLE 3 SUS440C Comparative Example 1 Target C amount (%) Target Camount (%) after sintering after sintering Steel type 0.75 0.80 1.001.20 Steel type 0.50 0.70 0.90 Molded 5.01 5.00 4.96 4.94 Molded product4.68 4.69 4.69 product density density Sintering 1230 — — 6.72 6.70Sintering 1270 5.44 6.23 7.38 tempera- 1240 6.88 6.91 6.88 6.93 tempera-1290 5.71 6.92 7.77 ture(° C.) 1250 6.93 6.94 7.00 7.10 ture(° C.) 13106.50 7.75 7.77 1260 6.97 7.00 7.19 7.52 1330 7.31 7.76 — 1270 7.03 7.127.61 7.63 1350 7.77 — — 1280 7.14 7.26 7.64 — 1370 7.77 — — 1290 7.247.41 7.63 — — — — — 1300 7.36 7.56 — — — — — — — — — — — — — — — C (%)0.84 0.86 1.04 1.24 C (%) 0.54 0.76 0.96 O (ppm) 130 60 42 34 O (ppm) 2114 20 N (ppm) 7 7 5 6 N (ppm) 3 2 13

TABLE 4 Example 1 Example 2 Target C amount (%) Target C amount (%)after sintering after sintering Steel type 1.30 Steel type 0.50 0.700.90 Molded product density 4.41 Molded product density 4.56 4.55 4.56Sintering 1240 6.34 Sintering 1290 5.88 6.12 6.44 tempera- 1250 7.10tempera- 1310 6.79 6.98 7.27 ture (° C.) 1260 7.68 ture (° C.) 1330 7.767.76 7.76 1270 7.69 1350 7.76 7.75 7.75 1280 7.70 1370 7.77 7.76 7.771290 7.70 — — — — 1300 7.69 — — — — 1310 7.70 — — — — — — — — — — C (%)1.25 C (%) 0.52 0.73 0.94 O (ppm) 11 O (ppm) 26 22 32 N (ppm) 7 N (ppm)10 8 7 Sintered hardness (Hv) 700 Sintered hardness (Hv) 600 640 310

TABLE 5 Example 3 Example 4 Target C amount (%) Target C amount (%)after sintering after sintering Steel type  1.30 Steel type  0.90 Moldedproduct 4.85 Molded product 4.85 density density Sintering 1230 —Sintering 1300 6.84 temperature 1240 6.37 temperature 1310 7.25 (° C.)1250 7.14 (° C.) 1320 7.58 1260 7.71 1330 7.83 1270 7.72 1340 7.83 12807.72 1350 7.83 1290 7.72 1360 7.79 1300 7.71 1370 7.77 1310 7.72 13807.75 C (%) 1.35 C (%) 0.94 O (ppm) 46 O (ppm) 11 N (ppm) 28 N (ppm) 9Sintered hardness 749 Sintered hardness 680 (Hv) (Hv)

As discussed above, in the alloyed steel powder for metal injectionmolding of the present invention containing Nb, the appropriatesintering temperature control range is greater than in the case ofSKD11, SUS420 and SUS440C. That is, while the appropriate sinteringtemperature control range is about 10° C. in the case of SKD11, SUS420and SUS440C, in the present invention it is broadened to about 50° C.,facilitating sintering temperature control and improving productivity.The sensitivity to C value after sintering is also weaker, and almostthe same sintering characteristics (temperature vs. density) areobtained with C values of 0.5 to 0.9%.

1. An alloyed steel powder for metal injection molding with improveddegree of sintering, consisting as mass percentages of 0.1 to 1.8% C,0.3 to 1.2% Si, 0.1 to 0.5% Mn, 11.0 to 18.0% Cr, 2.0 to 5.0% Nb, and aremainder of Fe and unavoidable impurities.
 2. An alloyed steel powderfor metal injection molding with improved degree of sintering,consisting as mass percentages of 0.1 to 1.8% C, 0.3 to 1.2% Si, 0.1 to0.5% Mn, 11.0 to 18.0% Cr, 0.3 to 1.6% of at least one of Mo, V and W,2.0 to 5.0% Nb, and a remainder of Fe and unavoidable impurities.
 3. Analloyed steel sintered body for metal injection molding with improveddegree of sintering, consisting as mass percentages of 0.1 to 1.7% C,0.3 to 1.2% Si, 0.1 to 0.5% Mn, 11.0 to 18.0% Cr, 2.0 to 5.0% Nb, and aremainder of Fe and unavoidable impurities.
 4. An alloyed steel sinteredbody for metal injection molding with improved degree of sintering,consisting as mass percentages of 0.1 to 1.7% C, 0.3 to 1.2% Si, 0.1 to0.5% Mn, 11.0 to 18.0% Cr, 0.3 to 1.6% of at least one of Mo, V and W,2.0 to 5.0% Nb, and a remainder of Fe and unavoidable impurities.